The present invention relates to a thermoplastic sheet material reinforced with a composite layer. The composite layer comprises the central reinforcing core of a three layer composite structure. The outer layers comprise a thermoplastic resin adhered to the core layer. The structural sheet material may be used in construction and manufacturing to provide a flat rigid outer surface for various appliances or vehicles and may be thermoformed into a variety of shaped articles.
In U.S. Pat. No. 4,044,188 there is disclosed a stampable thermoplastic sheet material reinforced with a central core of a multi-length fiberglass mat. A highly loaded thermoplastic resin also containing short reinforcing fibers in the form of sheet materials is laminated to both major surfaces of a glass mat or its equivalent to fuse and laminate the two sheets and the glass fiber web or mat into an integral sheet construction. Disadvantageously, the previously known structural sheet materials disclosed in the foregoing patent have been found to possess undesirable features. In particular, in applications involving thermocycling wherein the structural sheet material is repeatedly exposed to extreme low temperatures followed by heating to elevated temperatures, the highly expandable thermoplastic layers exert sufficient stress that failure of the central core layer is often the result. It might be thought possible to control the thermal expansion characteristics of the thermoplastic resin by including reinforcing fibers in the resin, but this approach has not proven effective in practice. In addition the presence of fibers in the resin layer may generate undesired surface asthetics.
An additional problem associated with structures of the prior art is the tendency of the resin to migrate into the glass mat under molding conditions thereby producing an undesirable surface appearance. To overcome this result various fillers may be included into the resin in order to increase the melt viscosity thereof. Disadvantageously the addition of such fillers can adversely affect the physical properties of the thermoplastic layer particularly the impact strength thereof. Also this procedure does not solve the fundamental problem of glass mat layer failure.
Composite papers are previously known in the art. A suitable technique for their preparation has been dislosed and claimed in U.S. Pat. No. 4,225,383. One commercial use for composite paper is as the backing layer of vinyl flooring materials. In these materials one surface of the composite paper is laminated to a polyvinylchloride resin layer.
It would be desirable if there were provided an inproved structural sheet material having improved resistance to failure due to thermal cycling and having improved dimensional stability without the use of reinforcing fillers or fibers.